1. Field of the Invention
The present invention relates generally to LCD displays, and more particularly, but not by way of limitation, to a method and system for driving LCD displays.
2. Description of the Related Art
Liquid crystal displays (LCD) are used extensively in electronic devices and displays. The LCD has become part of every day life, being included in devices have become digital in nature, such as automobile dashboards, computer monitors, radios, and watches.
Traditionally, LCDs have been used to display basic information, such as text, numbers, and symbols, mainly due to the limited capability of the LCD (i.e., on/off; black and white). However, more recently LCDs capable of displaying gray scale and color have become available. Further, technical advances in LCDs have provided the ability to use reflective polarizers within the LCDs to allow for screen printed images and colors to be selectively displayed. One such reflective polarizer is described in Ouderkirk et al., U.S. Pat. No. 5,828,488, and issued Oct. 27, 1998. An application of an LCD utilizing reflective polarizers is described in European Patent EP 0 825 477 A3, published Jun. 23, 1999, and issued to applicant Seiko.
An LCD is a passive device that does not generate light, but rather manipulates the ambient light that passes through it. There are many variations of LCD technology, but the most common of these is the field effect twisted-nematic LCD. To provide the reader with a basic understanding of LCDs and their operation, FIGS. 1 to 3B are provided and discussed hereinafter.
FIG. 1 is a layered representation of an exemplary LCD 100. The LCD 100 includes an upper polarizer 105 coupled to an upper glass layer 110. Beneath the upper glass layer 110 and coupled thereto is an (upper) electrode 115 that is generally transparent. A liquid crystal layer 120 is sandwiched between the upper electrode 115 and a lower electrode 125, which is coupled to a lower glass layer 130. A lower polarizer 135, which may be a reflective polarizer as described in EP 0 825 477 A3, as suggested above, is below the lower glass layer 130. A reflector 140 may also be located below the lower polarizer 135. Although not shown, a screen print may be located between the lower reflective polarizer 135 and the reflector 140. The screen print may show an image or simply reveal a uniform color when a segment is activated or non-activated depending on orientation of polarizers.
FIG. 2 shows selected aspects 200 of the LCD 100 that describe operability of the LCD. A light-molecule or source 205 that is oscillating (i.e., non-polarized) enters the upper polarizer 105. Because the upper polarizer 105 is polarized in a single plane, only light 210 having its direction vector in the same plane as the upper polarizer 105 passes through the upper polarizer 105, which generally results in a 50% decrease in light intensity.
Two states of the LCD are shown, (i) voltage applied and (ii) voltage not applied. In the first case, (i.e., voltage not applied), the light 215a is rotated in polarity by 90 degrees after passing through the liquid crystal 120. By not applying a voltage, or applying a voltage below a “turn-on” threshold, to the electrodes 115 and 125, the crystalline structure 120a of the liquid crystal 120 is twisted or rotated by 90 degrees. This 90 degree rotation causes the polarization of the light to be aligned with the lower polarizer 135 such that the light 215a passes through the lower polarizer 135. This light 215a is reflected off of the reflector 140 and a gray-on-gray image is displayed on the LCD as viewed through the upper polarizer 105. LCDs having a 90 degree twist of the liquid crystal, which are organic molecules, are, generally, twisted nematic (TN) liquid crystals. More recently, super twisted nematic (STN) liquid crystals provide for as much as 360 degrees of twist. The STN liquid crystals provide a much higher response to an applied voltage, thereby allowing for many more segments to be integrated in a display while still producing a high contrast display.
In the second case (i.e., voltage applied), the crystalline structure 120b of the liquid crystal 120 becomes aligned in the same direction (i.e., perpendicular to the electrodes 115 and 125) such that the light 215b is not twisted upon exiting the liquid crystal 120. Because the lower polarizer 135 is oriented perpendicular to the polarization of the incoming light 215b, the incoming light is blocked or absorbed by the lower polarizer 135 and is not reflected by the reflector 140. The image is seen on the LCD as being a “positive” image (i.e., black on gray) as viewed through the upper polarizer 105.
FIGS. 3A and 3B are exemplary LCDs 300a and 300b having seven segments for displaying a digit. In FIG. 3A, upper electrodes 115a-115g are each applied a voltage so that the digit “8” is displayed. In FIG. 3B, upper electrodes 115d-115f are each applied a voltage so that the digit “7” is displayed. The lower electrode 125 is considered to be a “common” so that a voltage differential is created between the segments connected to the upper electrodes 115a-115g having voltage applied thereto. It should be understood that the liquid crystal substantially sandwiched (i.e., within a segment, which is defined by common borders of the upper and lower electrodes 115 and 125) are affected by the root-mean-square (RMS) voltage applied to the electrodes 115 and 125.
Driving systems for LCDs generally include specialized circuitry that have standardized functionality. Two conventional approaches using digital circuitry have been taken by designers of driving systems for LCDs; a first approach is a fixed multiplexing approach, and a second approach is a pulse width modulation (PWM) multiplexing approach.
The fixed multiplexing approach operates on the basis of having a fixed number of lower electrodes or backplanes 125 connected to a driving system, where the driving system is configured to drive the upper and lower electrodes with predetermined voltages based on the number of backplanes to turn on and off the segments of the LCDs. A duty cycle is generated by the driving system to create an RMS voltage based on the fixed number of backplanes of the LCD. A limitation of the fixed multiplexing approach is that only two levels can be created on the LCD because the RMS voltage levels produced by the LCD driving system are fixed (i.e., on or off). Once a particular driving system (e.g., driver chip) and the number of backplanes of the LCD are selected or specified, a manufacturer of LCDs selects a liquid crystal fluid that operates within the range of the driving system. Those skilled in the art appreciate that a non-direct current (non-DC) voltage is generated by the driving system and applied to the LCD to avoid damaging the LCD.
Designers who desire gray-scale or color blends (i.e., voltage level changes) displayed on the LCD use pulse width modulation multiplexing. The pulse width modulation multiplexing approach operates on the basis of being able to drive an upper and lower electrode pair using pulse width modulation. One commercially available LCD driving system, SED1767, using conventional PWM is provided by S-MOS Systems, a Seiko Epson affiliate. This LCD driving system provides up to 16 gray-scale levels. However, this driving system requires many inputs, including gray-scale data bits to set gray-scale levels or duty cycles by the LCD driving system.
In general, the LCD driving systems used to generate various gray-scale voltage levels using conventional PWM to produce multi-level displays on LCDs are rather complex and expensive due to their unique functionality. Essentially, these specialty LCD driving systems have been developed for high-end commercial systems. Thus, consumer goods, such as watches, that are sufficiently driven by market considerations, such as price, are cost-prohibited from using LCD driving systems using conventional PWM multiplexing to generate multi-level displays (e.g., gray-scale and color) on LCDs. And, LCD driving systems operated using a fixed multiplexing approach, while inexpensive, cannot produce more than two levels on the LDC.